Where a MOS semiconductor apparatus using a MOS semiconductor device, such as IGBT, as an output-stage semiconductor device is employed for use with an inductive load, such as an ignitor switching circuit (for intermitting current through the primary winding of an ignition coil of an automobile, for example), the IGBT suffers from oscillation of its collector voltage. To overcome this problem, the inventors of the present invention proposed that a branch of series-connected constant-current device and resistor be provided between the collector and gate of the output-stage IGBT, as disclosed in laid-open Japanese Patent Publication (Kokai) No. 9-280147.
FIG. 19 is a circuit diagram (FIG. 1 of JP-A-9-280147) showing the configuration of the MOS semiconductor apparatus disclosed in the above publication. One of its output terminals (C) is connected to a primary winding of an ignition coil that is not illustrated. A branch in which a constant-current device 308 and a resistor 309 are connected in series is provided between the collector (c.sub.m) and gate (g.sub.m) of an output-stage IGBT 303. FIG. 21 shows output characteristics of this MOS power IC, wherein the horizontal axis represents the collector voltage of the IGBT, and the vertical axis represents the collector current. It is to be particularly noted that an unsaturated region of the constant-current device 308 is utilized to provide a characteristic that the collector current increases with an increase in the collector voltage, thereby to suppress oscillation of the collector voltage. In the above-identified publication, it is suggested to use a depletion type MOSFET or IGBT as the constant-current device 308, and fabricate or build this device into a part of the output-stage IGBT 303, but there is no specific description of such an integrated structure. It is also stated in the above publication that the constant-current device 308 may be in the form of a series power supply.
FIG. 20 is a cross-sectional view of a part of IGBT with which a depletion type and an enhancement type MOSFETs are integrated. The right-hand side portion of FIG. 20 illustrates an output-stage IGBT 320. An epitaxial wafer is generally used in which an n.sup.+ buffer layer 322 and an n.sup.- drift layer 323 are laminated on a p.sup.+ substrate 321, and a multiplicity of IGBT units are formed in a surface layer of the n.sup.- drift layer 323. On the left-hand side of FIG. 20, a depletion type MOSFET 330 is formed on and within a p.sup.- well region 333 that is formed in a surface layer of the n.sup.- drift layer 323. The middle portion of FIG. 20 illustrates an enhancement-type n-channel MOSFET 340 formed on and within the p.sup.- well region 333, which is not related to the principle of the present invention.
To provide the depletion MOSFET 330, an n depletion region 334, n.sup.+ source region 335 and an n.sup.+ drain region 336 are formed in a surface layer of the p.sup.- well region 333, such that the n.sup.+ source region 335 and n.sup.+ drain region 336 are located on the opposite sides of the n.sup.- depletion region 334. A gate electrode layer 338 is formed above the n.sup.- depletion region 334 with a gate insulating film 337 interposed therebetween. Source electrode 341 and drain electrode 342 are formed in contact with the n.sup.+ source region 335 and n.sup.+ drain region 336, respectively, such that the source electrode 341 is also in contact with the gate electrode layer 338.
With the arrangement as shown in FIG. 20, the constant-current device in the form of the depletion MOSFET 330 can be integrated with the IGBT on the same chip. As is understood from FIG. 19, the breakdown voltage of the constant-current device 308 is desirably equivalent to that of the IGBT 303 since these devices have a common output terminal (C). It is, however, extremely difficult for the lateral MOSFET formed in the p.sup.- well region 333 as shown in FIG. 20, to achieve such a high breakdown voltage as several hundreds of voltage. Accordingly, the semiconductor apparatus having the circuit configuration of FIG. 19 must use a discrete high-voltage constant-current device or a power supply.